No single word characterizes a great leader, the sort whose name and words end up in history books, who inspires cheers and tears, action and devotion. But, "charismatic" comes up a lot. Perhaps that's why a fair amount of research explores charismatic leadership, defined as "an attribution given by followers which focuses primarily on perceptions of inspiration." One new study, published in the Journal of Applied Psychology, argues that leaders are most likely to exude charisma, and be perceived as charismatic, when they and their followers are well-rested.

For the study, business school professors from the US and Pakistan looked at the "model of emotional labor and leadership," which connects charisma to expressions of positive emotion, aka "affective displays," such as smiling or speaking in a warm tone. Charismatic leaders, research suggests, both exude positive emotions themselves and provoke them in others.

Leaders can show positive emotions authentically — i.e., smile because what they're doing makes them happy — or they can fake it. In psych-speak, we fake smiles through "emotional labor," the "effort, planning and control needed to express a specific emotion." Emotional labor comes in two varieties. "Surface acting" amounts to smiling through misery. "Deep acting" is more like authentic fake happiness, to use a contradiction in terms. Rather than looking happy despite feeling sad, people make themselves feel good by, say, going to their happy place.

"In order to avoid beig perceived as less charismatic by their employees," study authors wrote, "leaders should reduce not only their own sleep deprivation but also that of their employees."

So, where does sleep come in? Well, past research shows that sleep-deprived people experience less positive emotions. And, faking a good mood (when it's necessary) requires people to overcome how they feel, or regulate their emotions, a process linked to sleep. Controlling emotional impulses appears to hinge in part on connectivity between two brain regions: the amygdala (the brain's emotional alarm bell) and the prefrontal cortex (involved in logical thought and central to keeping the amygdala in check). In the sleep-deprived brain, the regions show less connectivity.

The other half of the equation is followers' sleep, which also comes back to emotional positivity. For one thing, positive affect is thought to be somewhat contagious (electrifying leader, electrified followers). Based on the same emotional labor theory, researchers predicted that followers who experience positive emotional states while interacting with a leader will implicitly seek out an explanation for their good vibes, and they'll likely credit the leader. On a more basic level, research has found that people who are in good moods are more likely to "see positive attributes as likely." Also, studies depict sleep-deprived people as likely to understimate the impact of sleep deprivation on their emotional and cognitive capacities, so rather than attribute feeling shitty to their lack of sleep, tired, sad followers might project their feelings onto their leader.

Charismatic leaders, research suggests, both express positive emotions themselves and provoke them in others.

In the current study, researchers predicted that sleep-deprived leaders would exude less emotional positivity, in turn making followers less likely to assume a positive emotional state. And researchers see positive affect as fundamentally influencing perceptions of charisma.

To test their hypotheses, researchers ran two experiments, focusing, respectively, on sleep-deprived leaders and followers. Both experiments took place in the morning, after half of participants endured a night of sleep deprivation while the other half went to bed. Participants all filled out questionnaires about their previous night's sleep and their emotional states during the experiments. Researchers also determined the degree to which the leaders (but not followers) engaged in deep and surface acting (the two types of faking it).

For the leader experiment, 88 business students delivered mock graduation speeches. For the follower experiment, 109 (different) business students watched videos of the leaders' speeches and rated their charisma.

They found that sleep-deprived leaders engaged in less "deep acting" (that authentic breed of faking happiness). And a link emerged between deep acting and perceptions of charisma. So, the results suggested to researchers that leaders' sleep indirectly affected their charisma through their ability to espouse happiness via deep acting.

Sleep-deprived leaders are less able to display the emotions necessary to be charismatic and sleep-deprived followers aren't in the right emotional state to experience leaders as charismatic.

When either party is sleep-deprived, charisma suffers. Sleep-deprived leaders are less able to display the emotions necessary to be charismatic and sleep-deprived followers aren't in the right emotional state to experience leaders as charismatic.

"In order to avoid beig perceived as less charismatic by their employees," study authors wrote, "leaders should reduce not only their own sleep deprivation but also that of their employees."